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Multiple openings are used in culverts when your normal culverts are not big enough in size to cater the flow, and hence require multiple culverts joining together, or require multiple openings in order to cater for more flow.

Drawings of Pipe Culvert and Box Culvert #

Below is the example on keyplan of the culvert dimension with 1 opening on the left side and two openings on the right side. Given that the dimension of a opening is as follows:

D = 300 mm, W = 600 mm, Thickness = 70 mm

*The width of the dimension shown in the keyplan is inclusive with the material thickness*

This is how it looks like in 3D view:

*One opening (left) and multiple openings of culvert (right)*

Influence on The Calculation #

The number of openings affect proportionally the wetted perimeter and cross sectional area, and hence the capacity. Which means that 3 openings will get 3 times as much wetted perimeter, cross sectional area and capacity compared to just one opening of the same size.